Commit message: ax25: Fix UAF bugs in ax25 timers

There are race conditions that may lead to UAF bugs in
ax25_heartbeat_expiry(), ax25_t1timer_expiry(), ax25_t2timer_expiry(),
ax25_t3timer_expiry() and ax25_idletimer_expiry(), when we call
ax25_release() to deallocate ax25_dev.

One of the UAF bugs caused by ax25_release() is shown below:

      (Thread 1)                    |      (Thread 2)
ax25_dev_device_up() //(1)          |
...                                 | ax25_kill_by_device()
ax25_bind()          //(2)          |
ax25_connect()                      | ...
 ax25_std_establish_data_link()     |
  ax25_start_t1timer()              | ax25_dev_device_down() //(3)
   mod_timer(&ax25->t1timer,..)     |
                                    | ax25_release()
   (wait a time)                    |  ...
                                    |  ax25_dev_put(ax25_dev) //(4)FREE
   ax25_t1timer_expiry()            |
    ax25->ax25_dev->values[..] //USE|  ...
     ...                            |

We increase the refcount of ax25_dev in position (1) and (2), and
decrease the refcount of ax25_dev in position (3) and (4).
The ax25_dev will be freed in position (4) and be used in
ax25_t1timer_expiry().

The fail log is shown below:
==============================================================

[  106.116942] BUG: KASAN: use-after-free in ax25_t1timer_expiry+0x1c/0x60
[  106.116942] Read of size 8 at addr ffff88800bda9028 by task swapper/0/0
[  106.116942] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.17.0-06123-g0905eec574
[  106.116942] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-14
[  106.116942] Call Trace:
...
[  106.116942]  ax25_t1timer_expiry+0x1c/0x60
[  106.116942]  call_timer_fn+0x122/0x3d0
[  106.116942]  __run_timers.part.0+0x3f6/0x520
[  106.116942]  run_timer_softirq+0x4f/0xb0
[  106.116942]  __do_softirq+0x1c2/0x651
...

This patch adds del_timer_sync() in ax25_release(), which could ensure
that all timers stop before we deallocate ax25_dev.

Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
CVE: CVE-2022-1205
CWE: CWE-476
Vulnerability description: A NULL pointer dereference flaw was found in the Linux kernel’s Amateur Radio AX.25 protocol functionality in the way a user connects with the protocol. This flaw allows a local user to crash the system.

Project: linux
File: af_ax25.c

static int ax25_release(struct socket *sock)
{
	struct sock *sk = sock->sk;
	ax25_cb *ax25;
	ax25_dev *ax25_dev;

	if (sk == NULL)
		return 0;

	sock_hold(sk);
	lock_sock(sk);
	sock_orphan(sk);
	ax25 = sk_to_ax25(sk);
	ax25_dev = ax25->ax25_dev;

	if (sk->sk_type == SOCK_SEQPACKET) {
		switch (ax25->state) {
		case AX25_STATE_0:
			release_sock(sk);
			ax25_disconnect(ax25, 0);
			lock_sock(sk);
			ax25_destroy_socket(ax25);
			break;

		case AX25_STATE_1:
		case AX25_STATE_2:
			ax25_send_control(ax25, AX25_DISC, AX25_POLLON, AX25_COMMAND);
			release_sock(sk);
			ax25_disconnect(ax25, 0);
			lock_sock(sk);
			if (!sock_flag(ax25->sk, SOCK_DESTROY))
				ax25_destroy_socket(ax25);
			break;

		case AX25_STATE_3:
		case AX25_STATE_4:
			ax25_clear_queues(ax25);
			ax25->n2count = 0;

			switch (ax25->ax25_dev->values[AX25_VALUES_PROTOCOL]) {
			case AX25_PROTO_STD_SIMPLEX:
			case AX25_PROTO_STD_DUPLEX:
				ax25_send_control(ax25,
						  AX25_DISC,
						  AX25_POLLON,
						  AX25_COMMAND);
				ax25_stop_t2timer(ax25);
				ax25_stop_t3timer(ax25);
				ax25_stop_idletimer(ax25);
				break;
#ifdef CONFIG_AX25_DAMA_SLAVE
			case AX25_PROTO_DAMA_SLAVE:
				ax25_stop_t3timer(ax25);
				ax25_stop_idletimer(ax25);
				break;
#endif
			}
			ax25_calculate_t1(ax25);
			ax25_start_t1timer(ax25);
			ax25->state = AX25_STATE_2;
			sk->sk_state                = TCP_CLOSE;
			sk->sk_shutdown            |= SEND_SHUTDOWN;
			sk->sk_state_change(sk);
			sock_set_flag(sk, SOCK_DESTROY);
			break;

		default:
			break;
		}
	} else {
		sk->sk_state     = TCP_CLOSE;
		sk->sk_shutdown |= SEND_SHUTDOWN;
		sk->sk_state_change(sk);
		ax25_destroy_socket(ax25);
	}
	if (ax25_dev) {
		del_timer_sync(&ax25->timer);
		del_timer_sync(&ax25->t1timer);
		del_timer_sync(&ax25->t2timer);
		del_timer_sync(&ax25->t3timer);
		del_timer_sync(&ax25->idletimer);
		dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
		ax25_dev_put(ax25_dev);
	}

	sock->sk   = NULL;
	release_sock(sk);
	sock_put(sk);

	return 0;
}